The present invention relates to techniques for detecting the speed of an electric motor; and particularly to techniques which utilize the back emf of the motor to determine the speed.
Solid state controllers, such as those described in U.S. Pat. Nos. 4,996,470 and 4,978,887, have been developed to regulate the speed of an alternating current induction motor. These devices provide braking mechanism to decelerate the motor to stop in a controlled manner. The braking systems used in both the aforementioned patents have different modes of operation with a change from one mode to another occurring when the motor speed has reduced below a predefined level. Thus these controllers require a mechanism to measure the motor speed.
The controller described in U.S. Pat. No. 4,996,470 initially uses dynamic braking to slow the motor to a point at which cycle skipping can be applied to bring the motor and the device driven by it to an accurate stop. The dynamic braking mode involves sensing the polarity of the supply line voltage and the voltage induced by back electromotive force (emf) in the motor. When these polarities are opposite to each other, the supply line current is applied to the motor until the next zero current crossing occurs. This process repeatedly sends current pulses through the motor which produce a negative torque that slows its speed.
The dynamic braking has been used to reduce the speed of a motor to a point where cycle skipping can be employed to drive the motor at a slower speed. Cycle skipping is a power switching technique that generates a specific pattern of positive and negative pulses which applies current to the motor at an effective sub-harmonic frequency of the supply line frequency. The application of this pattern of current causes the motor to operate at a reduced speed. However, if the motor speed has not been reduced sufficiently when cycle skipping begins, the motor will remain synchronized to the supply line frequency component present in the pulse pattern and return to the previous higher speed. As a consequence, this braking technique requires a fairly accurate determination of motor speed in order to determine when the transition to cycle skipping can occur.
Previously the period between dynamic braking current pulses was used as an indicator of motor speed. The shorter the period between these pulses, the lower the motor speed. Although this speed determining technique can be used satisfactorily in many applications, in certain situations the characteristics of the motor and the driven load are such that the use of the pulse period causes a transition to cycle skipping too early and the motor returns to high speed. Therefore in these applications an additional mechanism must be provided to determine when the motor speed has been reduced to a point at which cycle skipping can be used effectively.
Other motor control techniques also require speed sensing and often employ tachometers or other electromechanical devices to produce an electrical signal indicative of motor speed. Many present day motor controllers incorporate a microprocessor, making it desirable to use the computational ability of that component to determine motor speed, rather than adding an electromechanical speed detection device.